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Why are airport firetrucks painted yellow green?
In my city, most of, if not all of the fire-trucks that serve city emergencies are red, just like this one:

But every time that I see an airport emergency that requires the fire-fighters, I see this kind of truck:

Is there a reason why the airport trucks are yellow-green and not red?

not all of them

in Italy they are red

Using the German term "Flughafenfeuerwehr", you get a quite different selection (although the more modern ones tend to have some lime in there). A thing to note are the blue flashing lights, which are standard on (and limited to!) German emergency vehicles. Looking at how German traffic reacts to a simple flashing blue light (and the two-tone "Martinshorn" siren), I always wondered why US emergency vehicles bother with that sound-and-lightshow of theirs. ;-)

@DevSolar You could blast Twisted Sister from our emergency vehicles while running a Pink Floyd laser show on top and nobody would move out of the way, but in true American fashion we operate on the principle that more/bigger/louder is better. (Just for fun try driving past one of our police cruisers at night on an unlit highway when it's got its roof-rack lit. I've had to pull over because their lights completely destroyed my night vision.)

@voretaq7: That's exactly what I wanted to hint at: Less might be more. You simply don't see any blue lights in traffic other than in emergency vehicles, and you don't hear that horn anywhere else either. So if you do see / hear it (which is comparatively rare), you get the heck out of the way because you know it's not some redneck in his pickup or the pizza deliveries, but real emergency. (And boy would you get in trouble if you'd mount blue lights on a pizza taxi...)

@DevSolar Not limited to Germany ^^ "Brandweerauto's" have them in blue as well. Sometimes the cars are yellow/white.

For the example in the picture, the vast majority of Miami-Dade county emergency vehicles are that color for visibility reasons, not just the ones at the airport.  Both the red and yellow-green varieties are ignored by the local population with the same intensity.

@DevSolar At least in the state where I live, blue lights are illegal for anyone but police vehicles and forward-visible red lights are illegal for non-emergency vehicles. While I won't say I never see other vehicles with blue lights, it is pretty rare and I don't think I've ever seen flashing ones here on a non-police vehicle.

@voretaq7 Yeah, the new LED light bars tend to be really high intensity, especially the blue ones. They really do hurt to look at. Even some of the new LED turn signals/brake lights have intensities that hurt to see, especially at night. Having said that, traffic moving for emergency vehicles is probably more of a regional thing. Here in the South, most people actually do get out of the way when feasible (though this probably has localized exceptions and the state of Florida doesn't count as the South. - lol)

Because yellow/green has been shown to be more visible than red, especially in low light conditions.  Yellow/green is also more easily identified by people with colour blindness and other visual disorders.
It's the same reason that safety vests are yellow/green.
This article gives some good references.

So now the question is, why are regular firetrucks painted red?

@DavidGrinberg Good question, not sure.  I've read that it's a mixture of tradition and because people associate red with fire engines.  There's psychology in there somewhere I'm sure.

Regular fire trucks in my city are painted yellow and have been for as long as I can remember. The link talks about people recognizing the color as  fire truck too.... the main factor is probably just tradition in whatever locality you're in.

The article you cite seems to be saying that it's the fluorescence that increases visibility, not the green-ness. This is unsurprising, since fluorescent objects are brighter than objects that merely reflect the same colour: fluorescence is the absorption of ultra-violet which is then re-emitted as visible light.

@DavidRicherby Which at night, is mostly missing.  Our eyes are very bad at seeing red at night.  It's why astronomers use red torches.  Yellow/green is the most visible colour in daylight and at night and overall; yellow is the most visible colour to the human eye. http://www.usroads.com/journals/aruj/9702/ru970203.htm

@DavidGrinberg Fire services in the US have been hotly debating the color of fire trucks for years. Many are no longer red because red is a lousy color for visibility, while others remain red because tradition outweighs many things in the fire service.

@Simon And they talk about retroreflective strips for night-time visibility. I don't see anything in the article you cite in your answer that says that non-fluorescent yellow-green would be better than red.

@DavidRicherby http://www.colormatters.com/color-and-vision/color-and-vision-matters, and many others obtained with a simple Google search.  The simple reason is that yellow activates both the red and green cones in the eye.

Re "It's the same reason that safety vests are yellow/green."  -- well I always spray-paint mine red, the better to be seen. Zinc chromate is yellow/green and is a known good color for priming, didn't think it had anything to do with visibility.

It's because it is the color specified by the Federal Aviation Administration and the only color of firefighting vehicle they will participate in funding.
Advisory Circular AC 150/5210-5D states

Aircraft Rescue and Fire Fighting (ARFF) Vehicles. Yellowish-green is the vehicle color standard.

and

APPLICATION.  The Federal Aviation Administration (FAA) recommends the guidelines and standards in this Advisory Circular for vehicles operating in the airport AOA. In general, use of this AC is not mandatory.
  However, use of this AC is mandatory for vehicles funded with federal grant monies through the Airport Improvement Program (AIP) and/or with revenue from the Passenger Facility Charges (PFC) Program. See Grant Assurance No. 34, “Policies, Standards, and Specifications,” and PFC Assurance No. 9, “Standard and Specifications.”  [emphasis in original]

Well, this explains why exactly Oshkosh's and their U.S. competitors' lineups use yellowish-green as default color. Good find!

to add acurved ball to the debate.in the uk,I worked as a locomotive driver and anyone trackside has to wear a dayglow orange hi viz jacket. for a trial period,they issued yellow/green jackets,and loco drivers had to give feedback.i can say,at certain light conditions,yellow/green was poor to see compared to dayglow orange,and the uks railways kept that colour. so perhaps fire trucks should be painted orange?

While Simon has told you about why ALL Fire Engines SHOULD better be yellow/green, not red, he did not tell you why all Airport Fire Trucks you have seen are yellow, and all normal Fire Trucks you have seen are red.
PBS's "Firehouse Primer" on the history of red fire engines:

Before firefighting was a paid profession, most communities were served by volunteer fire departments. These firemen didn't have much money to spend on upkeep, and at the time red was the least expensive color of paint. Red wasn't the only color used, however. Before it merged with the force in Manhattan, the Brooklyn Fire Department painted its apparatus a two-tone green to distinguish them from the red apparatus of the Metropolitan Fire Department. Today, there are still different colored fire engines, but red is the most common color out of tradition.

I don't know whether this is true or just an anecdote, but it's a nice story nevertheless.
While in the US, where fire fighting is a local issue, you will see yellow fire engines with some fire dept's (mostly in rural areas, I guess it's because you have long unlighted stretches of street there), in your native Brazil, the central government has decided to use red for all engines. This is the same in my native Germany, where the color mandated by law for fire trucks used on public streets has changed slightly over the years, from a dark red over a orangeish red to a retroreflecting orange-red. The Swiss, where fire fighting is a local issue as well, have mostly switched to yellow fire engines for visibility reasons. So I guess that you have only been to red-engine territory until now.
For airports, it's a different issue: Unless ordered otherwise, Oshkosh's (the US market leader) airport fire engine lineup is painted green/yellow. Rosenbauer (big player in Europe) on the other hand has a red lineup. While you can order other colors, most customers stick to  the default. So my best guess is that you haven't attended any plane evacuations in Europe until now.

Ah... so kind of like the color of tractors... John Deer green, Claas lime, Newholland blue...

No, no , no, no. Because they have eight wheels and four people on them, and four plus eight makes twelve, and there are twelve inches in a foot, and one foot is a ruler, and Queen Elizabeth was a ruler, and Queen Elizabeth was also a ship, and the ship sailed the seas, and there were fish in the seas, and fish have fins, and the Finns fought the Russians, and the Russians are red, and fire trucks are always “Russian” around, so that’s why fire trucks are red!

@Simon Nice anecdote, but I think you left out half of the story. Did you run into a comment's character limit?

Quoted from https://30yearspanther.rosenbauer.com/en/
The Rosenbauer Group is one of the world’s three largest manufacturers of fire-service vehicles and firefighting equipment, based in Leonding, Austria.

Two colors determine the appearance of the PANTHER in the world.
Fire-fighting vehicles are red, fire-red, which is the exact
designation according to RAL 3000 - almost everywhere in the world.
It's a different story for airport firefighting vehicles. About half
of the FLFs produced by Rosenbauer are painted "yellowish-green".
"Yellowish-green" is a slightly fluorescent, yellowish-green hue that
does not occur in nature and is therefore particularly well perceived
by humans. The special color is recommended in particular by the U.S.
Federal Aviation Administration (FAA) for airport fire trucks because,
according to studies, it ensures optimum visibility at all times of
day and night and in all weather conditions. Even with the classic
red, there are color gradations that enable better perception. For
example, the PANTHER is now painted not only in fire red, but also in
traffic red (RAL 3020), daylight fluorescent red (RAL 3024) or
luminous bright red (RAL 3026). Vehicles for airports in Great Britain
and Northern Ireland are also provided with the Battenberg pattern in
neon yellow/red, reminiscent of a checkerboard, which also serves to
increase visibility. The same effect is achieved by (retro)reflective
contour and warning markings, which are missing on practically every
vehicle.
Color exotics
However, there are also very special customer wishes as to how a
PANTHER should look. For example, vehicles at Riyadh Airport and at a
Canadian mining company are painted in plain white. The PANTHERs at
Newcastle Airport in the UK are painted half white and half purple,
because everything there, from the uniforms of the employees to the
front of the building, is kept in a uniform corporate design. The
PANTHER, which came to acting honors as Sentinel Prime in Transformer
3 "Dark of the Moon" and today watches over a nuclear power plant
(red/black), also has two colors. The same color scheme applies to the
vehicles at Toronto-Pearson and Seattle-Tacoma airports, while the
PANTHERs at Wellington Airport, for example, are painted in
yellow/black. Completely black was the prototype of the new PANTHER
6x6 at the Interschutz trade show in 2015.
Ultimately, it is a decision made by the fire departments or airport
operators as to which color their airport fire trucks should be. In
Europe, reds dominate, as they do in much of Asia and Africa. In North
and South America, the Middle East and Australia, vehicles painted in
the yellow color spectrum tend to predominate, with exceptions
confirming the rule here as well as there.
Blockquote
Individual grades
While the color of airport fire-fighting vehicles usually has a
functional meaning (good visibility in adverse visibility conditions),
the logos, lettering, names and symbols applied are more about
branding, the individual touch according to the operator's corporate
design. Every fire department and every airport (operator) has its own
ideas in this regard, and the motifs affixed or painted are
correspondingly diverse, ranging from the fire department or city coat
of arms to planes taking off (Stuttgart and Berlin airports) and
kangaroos (Airservice Australia) in the logo. Probably the most
spectacular motifs adorn the vehicles at Warsaw's Chopin Airport: on
the 6x6, a knight on his horse jumps out of a ring of fire; on the
8x8, it is the animal that gives the series its name, a black PANTHER.

Humans evolved in sunlight, which produces mostly yellow-green light frequencies. Therefore, we are most sensitive to the yellowish green part of the visible light spectrum. This is why safety vests are yellow-green. Airport firetrucks and new firetrucks are slowly starting to be painted yellow-green instead of the traditional red. There has been research which shows that firetrucks of this color are less accident prone than red ones.
Hope this was helpful (:


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How much is the minimum safe distance between two planes in-flight?
According to FAA regulations, how far should a commercial jet be away from another plane in-flight?

How about military planes? They get really close when in formations... Is there a minimum distance?

formation flight is a different beast than normal individual flights

In the 2nd picture, though the planes appear very close to each other, but that is because of the picture. In reality, planes flying in formations like this do not flight exactly on the same level. For example, see this picture, the planes following are higher.

@Farhan Not always true. There are different formations used in different scenarios, and sometimes vary to accomodate aircraft-specific characteristics. Some patterns with some aircraft are same-level. (Not necessarily in that particular picture though.)

Check out this Episode of Air Crash Investigation. It is about a mid air collusion caused by pilots not following ATC's directions. Though the crash took place on non US soil there is some information about flight separations during cruise and approach.

Also check out this video.

@Farhan Even if the planes are at different levels in that particular formation, they're still very close both laterally and vertically. And I've certainly been at airshows where the commentator has remarked that the planes performing some stunt are within a couple of metres of each other.

@caffinatedmonkey: I love your typo! Mid air collusions make me ponder the possibility of pilots secretly communicating vile schemes without ATC knowing about it...

@RobVermeulen Hmm... So, do we now need to investigate whether Trump Force One and any Russian government aircraft were in nearby airspace during the election? ;)

The question specifically asks about FAA regulations, so I will try to provide specific citations.
Summary
For a commercial airliner (as the question asked), separation will usually be at least 3 miles laterally, or 1,000 feet vertically. In the enroute environment -- at higher operating speeds above 10,000 feet and based on the type of Radar and distance from the antennae -- a 5 mile rule is applied laterally. This is true in most but not all situations. There are exceptions: see below.
Note also the "or": it is allowed (and in fact rather common) for two jets to cross paths at the same moment, with one 1,000 feet directly above the other. The 3 miles is only required if the jets do not have at least 1,000 feet of vertical separation.
Pilots
The regulations do not set a specific distance that pilots flying under VFR must maintain from each other. There's just the following blanket statement:
§91.111, Operating near other aircraft: (a) No person may operate an aircraft so close to another aircraft as to create a collision hazard. (b) No person may operate an aircraft in formation flight except by arrangement with the pilot in command of each aircraft in the formation.
In addition, there is the catch-all "careless or reckless operation" clause:
§91.13, Careless or reckless operation: No person may operate an aircraft in a careless or reckless manner so as to endanger the life or property of another.
Pilots flying under IFR are subject to the same kinds of rules, and are still responsible for maintaining separation from other traffic when in visual conditions. The rules are the same: the pilot is given the discretion of deciding what a "safe distance" is.
Actual distance limits only come into play for the air traffic controllers, who are responsible for providing minimum separation distances between IFR traffic.
Air Traffic Controllers
The set of FAA regulations for air traffic controllers is FAA order 7110.65. These regulations are quite complicated and there are lots of exceptions and corner cases for various scenario. I'll try to give only the most basic rules here, but note that a full reading of 7110.65 would be necessary to understand all the minutiae.
Vertical separation:

4-5-1 Vertical Separation Minima. In general, 1,000 feet for IFR traffic (with exceptions).
7-7-3 Separation, 7-8-3 Separation, 7-9-4 Separation. 500 feet for VFR aircraft (with exceptions).

Lateral separation:

5-5-4 Minima. In general, 3 miles in the terminal environment and 5 miles in the enroute environment (with exceptions).

Things get more complicated in the following scenarios, and exceptions apply to them:

Oceanic environment (7110.65 Chapter 8).
Approach environment, especially at large/busy airports with parallel runways and special approach monitoring radar. (7110.65 Chapter 3, Sections 9 & 10).
Nonradar environment (anytime the controller is not able to "apply radar separation") (7110.65 Chapter 6).

For these scenarios the regulations are rather lengthy and complicated, so I would simply refer you to Order 7110.65 in general.
Military
When flying in airspace controlled by ATC, military pilots must follow the same rules as civilian aircraft. ATC will keep them separated according to the rules above. However, there are two main exceptions to this.
When flying in formation, such as for training, aerial refueling, or intercepting aircraft, they are much too close for ATC to provide safety. In these cases, ATC will allow the pilots to fly under MARSA (Military Assumes Responsibility for Separation of Aircraft). In the case of interception, if the fighter pilot is not able to see the aircraft they are intercepting before the minimum separation, they must break off. They can only fly under MARSA if they can actually ensure visual separation.
The FAA can also define an MOA (Military Operations Area). Once military aircraft enter this area, ATC is no longer responsible for them and they are under MARSA. When the aircraft leave the MOA, ATC resumes control. They can still be under MARSA for a formation, but ATC will keep that formation separated from other traffic.
In tactical formation flying, there is no limit. The Blue Angels routinely fly with only 18" separation between the wing tip and the cockpit canopy. Combat aircraft fly a variety of different formation patterns at varying distances, and pilots are extensively trained on each pattern and what the appropriate/safe distances are. (For example, a fingertip formation allows for much closer flying than a line-abreast formation.)

Excellent answer!  I would suggest adding a paragraph about visual separation between aircraft since it is a commonly used procedure.  I would also suggest making it more clear that the 1,000 ft separation is for IFR-IFR traffic.  It is common for IFR-VFR to have 500 ft separation because that is how the "normal" flight levels are setup.

I believe the part about military only applies to US and the rules are very different in different countries.

I have a friend who was the thunder birds' commander for a few years.  You can see, in some Go Pro videos of his, that their wing-tips would actually bump into each other sometimes.

@SpongeBob There is absolutely no way they had a minor midair and kept flying. A mishap report would be filed and both crews would likely be investigated. Bumping a wingtip means that you are vertically out of position.  You should never be at or above leads altitude flying parade.

@SHAF - If you say so.

@SpongeBob "Bumping wingtips" could cause catastrophic damage and severely reduce laminar flow over the wing.  That would have unpredictable effects on the rest of the jet and lead to horrible accidents.  What you likely saw was a very tight formation with the wing and lead staggered by altutude making it appear they were touching.  The AF is more restrictive than the Navy, and if i touched a wingtip it would be an immediate knock it off, followed by a controllability check and a land as soon as possible.

@SHAF, I'm not arguing with you.

@SpongeBob Sorry, my kids are pissing me off and I'm feeling particularly combative.

@SHAF, Haha.  Fair enough.  Better take it out on me than them! (Seriously)

Would it be a normal occurrence for an intercepting fighter to not be able to see what he was intercepting outside of 3-5 miles? GA craft tend to be painted in high-vis color schemes, but I could imagine a drug run made with a 172 painted in forest camo with a sky underside. Even with radar telling you where to look, it can be difficult to spot a plane that doesn't want to be seen.

@KeithS Even in high-visibility paint, an airplane can definitely be hard to see beyond a few miles, even on a clear day. Hazy air and bright sun can make it hard to spot even close-in. But military interceptors have onboard tactical radar systems, and ATC (civilian or military) can provide intercept vectors from the ground. Your drug runner may have pretty paint, but he probably isn't going to have low-observable (stealth) technology. If he does, well, we're beyond the scope of this Q/A to be sure :)

Two planes travelling around 0.85 mach at their peak altitudes, how much time do they have to react for a 5 mile lateral gap? I am just trying to think if the gap is enough.

0.85 mach is around 490 knots TAS at 35,000 feet. If they're closing head-on, that gives a closing speed of 980 knots, and the 5 miles will be covered in about 18 seconds. Not very much time to react, but at this point ATC has already messed up, and TCAS becomes the next line of defense: both planes will receive a TCAS RA (Resolution Advisory), and as long as both pilots respond immediately (as they are trained to do), avoidance will be successful.

You forgot RVSM for vertical separation. Above FL410 separation is 2000ft for RVSM capable aircraft. Also for non RVSM capable aircraft, vertical separation is 2000ft from FL 290 to FL 410 and 4000 ft above FL 410

you have 3 separations for commercial flights

vertical separation: two aircrafts flying in the same direction will be separated by 2 flight levels (FLs), equivalent to 2000 ft. The intermediate FL is occupied by planes flying in the opposite direction. Each aircraft has thus 500 ft above and below the assigned altitude of free airspace.
lateral separation: the airways are defined as being 10 nautical miles wide, 5 for each side
longitudinal separation: I have no detailed information on this for the cruise, I hope someone else can fill the gap. During approach it depends on the size of the preceding aircraft.

you can write an entire book on longitudinal separation and still not be done with it: depending on whether it is radar or procedural separation, VOR, DME, Mach number tehnique, cruise or approach service. About 10 minutes or 80NM for cruising and between 2 to 6 mins or 5NM to 2.5 NM on approach is a good rule of thumb

@Radu094 Lol, absolutely true. In fact an entire book has been written about it... FAA Order 7110.65 :)

With formation flights it is as close as the pilots feel comfortable with in the circumstances. On clear days and in level flight brushing wingtips is allowed as long as you don't endanger the other planes (which actual brushing would do).
For normal (commercial) flights vertical separation of 1000 feet is maintained when passing each other.
for planes incoming for landing there is 4-8 NM radar separation depending on the size and weight of the planes.

If by brushing you mean touching, then no, that's never allowed and is a great way to die.  Also, military flights routinely execute section approaches in bad weather where the wingman flies in parade and the lead shoots the approach.

I think a more precise definition to the phrase "brushing wingtips is allowed" is required here.

Yeah, I down voted your answer due to the "brushing wingtips is allowed" remark...

It truly depends on your operation (IFR/VFR) - The following answer is in regard to IFR operations in class A airspace (+18,100ft)
Below FL290 (29,000ft)
Vertical separation of 1,000 ft is required
Between FL290 and FL410
Vertical separation of 2,000 ft is required... but under RVSM (Reduced Vertical Seperation Minimus) regulations separation can be reduced to 1,000 feet if the aircraft is approved for RVSM operations.
Above FL410 the minimums are bumped back up to 2,000ft regardless of RVSM approval.
See: AC 91-85A for more information regarding RVSM

Two answers here are "mostly" right, but not exact. What's relevant is IFR vs. VFR, not whether or not it's "commercial". In IFR it's 1000 ft vertically and 3 miles below 10,000 ft and 5 miles above 10,000 ft. This applies to non-commercial flights as well, such as part 91 flights, this has nothing to do with the flight being "commercial". And, a commercial flight can also be in VFR, not every commercial flight is IFR. Every airline flight is IFR (part 121), but you can't equate airline operations with commercial flights. I can do a commercial flight under VFR, and I can have the above-mentioned minimums on a non-commercial flight.

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How do modern turboprops avoid slow spool up of early jets?
I was reading about modern crop dusters that are blessed with 600 or more horsepower
turboprops and have seen them do amazingly effortless high angle of bank turns near the ground.
It seems that a slow rate of power increase would make this type of flying much more dangerous. I had imagined using air bleed gates to close and raise combustion chamber pressure instantaneously, along with more fuel.  This would involve an overdesigned compressor.
Is this how it is done?

Two ways:
As Dave pointed out above turbopropeller engine cores, like all aviation gas turbines, do have a spool up lag.  This is mitigated a couple of ways.
In a free turbine turbopropeller engine, the gas core spool is independent of the power turbine, allowing it to spin up much faster, and subsequently increase the amount of power delivered to the power turbine much faster.
In direct drive turbopropeller or turboshaft engines, the engine - and the propeller - operate at a near constant speed with increase or decrease in power provided by altering fuel flow and adjusting propeller pitch for the load.  Direct drive engines like the Garrett TPE-331 have a near instantaneous power response to power lever commands.  The downside to these designs are that they can be extremely treacherous in the event of an engine failure without an autofeather system linked to a negative torque sensor.  They are also very noisy on the ground.

Appreciate the info.  With direct drive the fuel/air mixture may be off while the engine responds to increased throttle.  A little rich will temporarily make for more "smoke".  It would seem best to lag the prop pitch change slightly behind power increase to help it speed up faster.

The 331 is very efficient.

They don't avoid it but they do try an mitigate it. Turbo props do have jet lag, even modern jet engines have some lag in spool up, granted its not what they once had.
At least some of the lag comes from the sheer mass of the core, production turbo props like the PT-6 tend to me much smaller than your average jet engine and thus may respond faster.
As for crop dusters. Keep In mind most are build to haul a lot of load. When flying empty even at partial power the aircraft has plenty of power to make quick maneuvers.

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Could placing a fan jet further in front and closer to the wing create undesired pitch motion from its exhaust?
So far in discussion of the Max 8 people have mentioned CFM LEAP cowling size and 737 tail design as possible factors in the need to develop a MCAS system to control tendency to pitch up at high AOA.
Since the tail has much more leverage than the wing (much longer lever arm), is it possible that the high bypass exhaust blast is affecting the tail at high AOA by pulling more downwash onto it by Coanda effect? At first I though the may be affecting the back of the wing, but then realized the 737 Max 8 is very close coupled (tail close to wing) and may be susceptible to stronger downwash effects.
I wanted to be sure before suggesting a larger horizontal stabilizer or considering lengthening the fuselage.  Is it possible?  Has it ever been seen before?

I haven't seen that mentioned in any of the multiple articles I have read in any of the aviation areas.

I seriously doubt the engine exhaust hits the horizontal stabilizer except possibly when fully stalled. Otherwise, the wing downwash follows approximately the extended chordline and takes the engine with it safely below the horizontal stabilizer, which is significantly higher than the wing.
The one thing I've seen mentioned was change in the centre of gravity generally decreasing the longitudinal stability—which is most pronounced near stall, because the centre of pressure moves slightly forward with increasing angle of attack (until stall, where it moves abruptly aft all the way to midchord).
The other possible effect is that the cowling generates vortices—possibly with help of vortex generators needed to avoid stagnation in the gap between the wing and the engine—that go over the wing at high angle of attack and increase the suction peak near the leading edge and making the forward shift of centre of pressure more pronounced.
The only point regarding tail I've read is that it is not large enough to allow moving the allowable range of centre of gravity forward to improve the stability ‘naturally’.

"Naturally" or passively, unless the larger diameter airstream produced by the high by pass jet is strong and close enough to pull tail towards it like a vacuum aspirator.  I wanted to check this before clamoring for a larger Hstab.  If airflow under power checks out for all possible AOA (no surprises such as blanking the elevator) this may be an easy fix for them, as they have many orders.  If not, perhaps an enhanced version of the 717 tail, if not the entire 717, may be needed.

@RobertDiGiovanni, larger tail in itself would not fix anything. It would only allow moving the centre of gravity further forward, which would, but it would also create a bit more drag. Also designing new stabilizer, even if it is just slightly scaled up version of the previous, is a complex process that would take quite long. Redesigning as few parts as possible was the point of the upgrade.

In other words, cutting corners.  Notice the larger tail rebalances the CG back to its original point as well as aerodynamicly stabilizing the aircraft.  I would not worry about drag on a regional jet.  A 717 tail/737 LEAP engine mount hybrid may be in the works.  $$$, but it might save their reputation.  We'll see.


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Can surface roughing effects on an airfoil Reynolds number be quantified?
I am building balsa free flight gliders with a chord of 0.10 meters and a velocity of around 3 meters per second.  This works out to a Reynolds number of around 15,000.
The wings are around 1 meter in span, and are made with the wood grain running lengthwise.  They are thin undercambered (no bottom covering) with maximum thickness of 7 mm at 30% of chord from leading edge.
Because of the roughness of the unfinished grain, is my Reynolds more like 50,000, or am I still down in the range where a flat plate might be better?

Roughness has no influence on the Reynolds number. It will, however, help to trigger a laminar-turbulent transition. That is the reason for dimples in golf balls. Also, roughness will increase the friction drag in turbulent boundary layers.

Flat plate friction diagram (picture source). Note the horizontal lines: They show that drag does not drop any more with increasing Reynolds number once the surface roughness exceeds a certain value. Roughness is given here relative to the chord length as $\frac{\epsilon}{l}$, with $\epsilon$ being the mean height of the roughness and $l$ the chord length.
The diagram only starts at Re = 100,000, so it is clearly outside the range of your application. If you sand your wooden surface with fine grit sandpaper, I don't expect that the remaining roughness will increase drag. At those Reynolds numbers it might be advantageous to add a trip wire in order to induce a transition to turbulent flow – see the golf ball example.

Frisbee grooves may be a bit closer.  Roughness to chord length ratio interesting way of trying it, as well as location on the airfoil.

What's causing the increasing in drag with Re for the low surface roughness lines?

@Jimmy Don't confuse "increasing drag" and "increasing the drag coefficient". For a given structure, lower Re means lower velocity and the drag coefficient is multiplied by $v^2$ to get the drag force. But for laminar flow the drag force is proportional to $v$ not $v^2$, so the drag coefficient used in the conventional formula appears to be bigger at low Re even though the drag force is less.

@alephzero I didn't confuse. 1e6 Re is a standard wind tunnel Re for engineering design. No one will say the drag is linear to V.

What I'm getting is that larger roughness to chord ratio is "completely turbulent" at lower Reynolds, therefor "transitional turbulent" at a lower airspeed, where drag reduction occurs.  This may have been what Kline-Fogelman were after, but their "disrupter" steps may have been too large.

@RobertDiGiovanni and were on the wrong side. The graph is for a flat plate while you need to look at a cambered airfoil with an angle of attack. The pressure distribution is different and so are the needs for the boundary layer.

@Peter Kampf well, I'm starting to see why they didn't bother to go below 10^5 for full scale aircraft, some very interesting things going on at lower Reynolds (even down to insect wings).  All part of an amazing spectrum.


text:
What provides a higher lift to drag ratio, a thick laminar wing or a thinner wing with a vortex generator
Recent readings have shown the benefits of "boundary layer tripping devices" or vortex generators for reducing drag and maintaining attached air flow at higher angles of attack.
It seems these generators effectively increase the width of the wing by creating an energized
"fence", which further deflects the free air stream, almost acting as a lubricant to reduce friction drag on the surface of the wing and increasing the "bend" of the airstream, thereby increasing lift.
But generating these vortices comes at the expense of increased drag to create them, though they (and slats) could be made to be retractable.
Is there a clear advantage to either a thick laminar wing or a thinner one with vortex generators?  The Reynolds number for the application would be around 200,000 to 500,000.

You confuse turbolators with vortex generators. And then you misunderstand the function of vortex generators.
Turbolators trip the boundary layer and change laminar to turbulent flow in order to enable the boundary layer to remain attached over a steep pressure rise. That is definitely required at the Reynolds numbers mentioned (200,000 to 500,000) in order to maximize performance.
Vortex generators work in a fully turbulent boundary layer and re-energize it by mixing more of the outer flow into it.
Both increase the boundary layer drag but help to prevent early separation. Only over the range of that prevented separation they reduce drag.
The answer to your question depends on the Reynolds number. At 1,000,000 and above, the laminar wing should be better and if you add boundary layer tripping at the right chord station, it also wins down to 200,000.

From what you describe it seems then one could have both, with the turbulators closer to the leading edge.


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What effect did the AWACS radar dome have on the stability of the 707?
The large disc shape of the AWACS radar dome, and its attachment pylons to the fuselage, above and behind the wings, seem to have the potential to actually improve the pitch and yaw stability of the 707 carrier aircraft in spite of its ungainly appearance.
In light of the current debate over placement of increasingly large fan jet nacelles, could stability data from the AWACS radar dome configuration inspire improvements in current designs?

The nacelles suck in air at low speed, so the forces on their forward edges are much greater than those on an ellipsoid in free flow. Yes, the radome helps with directional and pitch stability but increases dihedral effect. However, given its large drag from the rounded trailing edge, nobody will seriously add a radome just to improve stability. Look at the Beech 1900 for ways to add some stability on the cheap. Hint: Place additional surfaces as far back as possible.

@Peter Kampf perhaps they will take another look at the Sonic Cruiser design, using fan jet nacelle area combined with horizontal stabilator area for improved stability.  For lower subsonic, bring the wing forward and straighten it out a bit.  They could turn the fan nacelle issue into a positive with a little bit more effort.  A triplane yes, but a good one.


text:
Why were the first airplanes "backwards"?
In the question Is the location of an aircraft spoiler really that vital? the accepted answer states "Surfaces behind the CoG act as stabilisers, keeping the nose pointing forward. An aeroplane has vertical and horizontal tail surfaces at the back just for this purpose."
I agree that this seems straightforward, to a layman (me). So why then were so many of the first aircraft built 'backwards'. Taking a look at the Wright Flyer
Image (C) Bay Images
as an example. There are many other examples from the earliest days of aviation. Why did many put the elevators up front, thereby destabilizing the whole thing?

Could you provide some of the many other examples?

@zymhan: 14bis by Santos-Dumont, Voisin - between 1903 and 1907 all successful motorised aircraft had the horizontal "tail" up front.

That's a huge horizontal stabiliser!

related: Why did moving the CG aft on the Wright Brother's plane improve handling? The answer explained is was not stable but flyable.

Speaking of why, it's because the maker of this plane doesn't quite understand aerodynamics (not many people do at the time)

I feel like there's something to be said for having the moving parts in front of the pilot where he can see that they're working reliably, at least when you're still in the experimental stages of design like that.  Also, there's an analog to the most popular mode of transport of the time, that being the horse-drawn carriage.  The things pulling and steering the vehicle (horses/elevators) were located at the front, under direct control of the driver/pilot).  This would be the natural evolution of that idea.  It took much experimentation to learn that other arrangements were superior.

The Wright Brothers stated explicitly that having the horizontal control surfaces in front of the pilot enabled the pilot to see what was going on much easier than with a so-called conventional tail.
Notable is that the most successful private aircraft, what we would call a homebuilt, was the Pour-du-Ciel, the Flying Flea. This has also the pitch controls on the forward wing, and conventional rudder, but no ailerons, or similar systems.

Many modern fighters have pitch controls on a canard wing, a small forward wing in front of the main wing. Like the SAAB Gripen, and many Sukhoi aircraft.
Most efficient, according to designers at Boeing, is having three surfaces, one forward for pitch control in cruise, the main wing, and a tail for maneuvering loads. Typical of this kind of design is the Piaggio Avanti.
Stall, when the wing loses its lift, is difficult to handle with conventional aircraft, but much easier with a canard type of aircraft, unless the Center of Gravity is too far back, as with John Denver's Long Eze!

"Backwards" is relative, there are modern aircraft that have forward placed elevator i.e. canard designs that fly quite successfully

(source)
As for why its not more popular you can read up more on that in why are there no production canard GA aircraft? and Why is the Tu-144 the only commercial airplane with canard configuration? as well as in the answers to lots of 'canard' questions on this site.
As for why the Wrights did it this way, NASA offers an explanation

The placement of the elevators at the front of aircraft is rather
  unique for the Wright flyer. Modern aircraft typically have the
  elevator at the rear, attached to the horizontal stabilizer. The
  Wright's placed their elevator at the front to provide protection to
  the pilot in the event of a crash. (The pilot of this aircraft lies
  next to the engine on the lower wing.) But there is also a static
  performance advantage when the elevator is placed forward. Lifting
  wings have a natural tendency to flip tail over nose because of the
  way the pressure is distributed.

So they were not necessarily backwards so much as different considerations were taken under advisement during the design.

Unique? And what about the 14bis by Alberto Santos-Dumont? In 1906, 100% of flying machines were canards - hardly unique if you ask me!

Lilienthal had a special "Prellbügel" (bumper bar) fixed to his gliders to absorb potential crash loads. That saved him several times. Unfortunately, on that fateful day in August 1896 he left it off …

Shockingly bad grammar from NASA there.

I am not sure you are correct that most early planes placed the elevator at the front. For example, Otto Lilienthal's gliders had the tail at the rear.
The Wright brothers were strongly influenced by Lilienthal's work, but were also very anxious to avoid his fate, and believed they would obtain control he lacked by placing the elevator at the front (amongst other things).
It turned out that their Flyer was in fact very unstable, and difficult to control well - but controllable enough.
They also apparently felt that a tail at the rear would be more susceptible to landing damage.

Alphonse Penaud and Lilienthal had the tail in the back, correct. But Santos-Dumont in 1906 and Gabriel Voisin also put the horizontal tail in the front (the Voisin designs had two, one forward and one rear, to be doubly sure they can be stabilized). So in the short period from 1903 to 1907 all aircraft had the tail in front.

"One forward and one rear, to be doubly sure they can be stabilized" Yes, it was the early days.  Unfortunately for aircraft pitch stability, the forward will cancel the rear and we're back to square 1. We must remember "tail" functionality regarding directional and pitch stability MUST be aft of the CG.  The elevator (essentially a pitch destabilizer) can be placed fore or aft.

It's not destabilizing to put the elevator or horizontal "tail" in front, as long as you place the CG sufficiently forward that a large portion of the wing itself is well behind the CG and effectively acting as a tail.  The fact that the forward elevator or canard is trimmed to generate positive lift, is what allows you to place the CG well forward in this manner.

Now, another related question-- why did the Wrights place vertical "curtains" in front of the CG (forward of the pilot) on some of their aircraft? It never made much sense to me.

Also, the answer could note that the Wrights configuration tended to avoid a bad nose-drop in a stall.

Also this answer could reference the Wrights' concerns over Lilienthal's experience as has been nicely pointed out in another answer.

"as long as you place the CG sufficiently forward" – yes, but the Wrights forgot about this part.

@quietflyer: Having an stalled aircraft assume an attitude where it can't gain airspeed is a guarantee that almost any stall will force an involuntary landing, but limit the speed at which the landing will occur.

@supercat "involuntary landing" - that wins euphemism of the day.

Remember that the aviation pioneers were inventing the skills required to fly while refining their designs.  It would be a great help to actually see the position of the elevator while trying to relate its movements to the results.  We relate control pressures (which we sense in our hands and feet) to the aircraft movements to sense how we are doing and we learn that from instructors and through practice on well designed aircraft.  The Wrights were trying to figure it all out as they went.

Its probably worth remembering that the majority of design work in those days was trial-and-error rather than mathematical analysis

I was recently at the Smithsonian Air & Space museum and watched a video about this. They used a ribbon tied to the front of the aircraft as an early instrument. A sense of balance, like that required to ride a bicycle, was a large part of early flight!

They were not backwards, they had a huge horizontal stabiliser at the aft section!.
Angular accelerations are relative to the CoG. If there is only one aerodynamic surface, it must be behind the CoG in order to self stabilise. If there are two of them, like in the plane through the Y-axis, basically the same stipulation holds: that the total centre of lift is behind the CoG.


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Does the DC-3 passenger airliner have slats?
The DC-3 is a very popular and widely used aircraft, yet has low speed tip stalling risks due
to the design of its wing, which at the time of its development represented the leading edge in drag reduction for larger and faster mono-wing aircraft in the 1930s.
Specifically, the swept and tapered wingtips, at high angles of attack, would stall first.
Did the original design, or later modifications, include slats to remedy this issue?
Is there any way to reduce this risk other than retractable slats?

Although it's technically possible, you aren't going to find slats retrofitted to a DC-3 (nobody would ever pay for it).
But yes, in theory you could improve the DC-3's stall behaviour and single engine performance very cheaply with Vortex Generators which can be thought of as a "poor man's slat".  VGs increase stalling AOA by between 1/3 to 1/2 of what you get with a slat (say, about 18-20 deg, vs 25 with slats) depending on the airfoil.
VGs have huge benefits on light twins, reducing stall speed, taming stall behaviour, improving aileron response, and often reducing Vmc to the stall when installed on the fin.  They are quite miraculous, although they can be a headache because they are usually glued on and if the kid you pay 20 bucks to wash your plane knocks off too many, you are grounded until you fix them.
I can't find anybody that sells a VG kit for the '3 although it does seem to be possible to install them on a wing with boots. Possibly the DC-3's airfoil requires VG placement far enough forward that the boots are in the way (each airfoil has a chord wise sweet spot and if they are too far aft they don't do anything).  Or maybe nobody has thought of doing it yet.

Just got done viewing the NACA film on this.  They used a sharpened leading edge installed between engine nacelles and fuse to create a warning "buffet". Also they seemed to apply a little INSIDE aileron in the recovery. This made sense as the aileron extends pretty close to the wing tip. I surely would let go of the elevator.

@RobertDiGiovanni  Note that what I'm describing is an entire array of VGs across the leading edge.  I've thought of making a set and putting them on my own plane (a homebuilt) but I need to find out the proper chordwise location for my airfoil.

I have heard 6-10% of the chord from the LE, so you were right about possibly being too close to de-icing boots.  Of course, with your own plane little steps might be best.  I might try the sharpened leading edge on the LE closest to the fuse, and VGs at the wing tips.  Maybe a little washout too.  But that old plane looks real good to me.

Greatest engineered conveyance in history in my opinion.  Can you think of any other complex transportation machine, designed 85 years ago, that businesses still, two decades into the 21st century, still purchase as a tool, to put to work making money as opposed to using as an expensive toy to show off.  I can't.


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Could a tail rotor serve as a spin breaker for winged aircraft?
It seems that yaw, more than pitch or roll, creates the greatest wing asymmetry in a spin.
If the spin progresses to the point where it cannot be broken, some aircraft are equipped with parachutes to try to stop rotation and to get the nose pointed down. Some cannot be saved.
Tail rotors work well in helicopters, and are part of the design to control the (spinning wing) rotor yaw, and, such as the 180 knot MH-65 Dolphin, have the rotor built into the tail to save drag.
Could this work as a safety feature for GA aircraft?

Probably unrealistic, think about the weight and complexity.  Some sort of one-shot rocket pack might work, but probably better for deploying a recovery parachute.  In some future fixed wing multi-propellor electric design you could more readily have distributed thrust but you're also more likely to see software that tries to prevent getting deeply into trouble in the first place.

That would be a monstrosity in search of a cause.  Even airplanes not certified for spins will recover all by themselves, or with correct inputs, if you don't hold them in the spin for several turns.

If?  Perhaps if we had a number on the torque required to break a spin, an ELECTRIC (not rocket) may be possible.  If reading about a Cessna 152 going flat (they really had to try) it was a thought.

@Robert DiGiovanni: Re "they really had to try", why add weight and complexity to airplanes in order to try to save a very few idiots from the consequences of their own stupidity?

At least one of two possible devices is regularly added before spin testing:

Jettisonable mass, for example by fixing a sandbag to the tail, or
a spin chute.

Spin chutes do what you propose in yaw, but what they mostly do is to pull the tail up so the angle of attack over the whole wing is reduced. This is equally achieved by jettisoning that extra mass at the tail: Now the more forward center of gravity will add a nose-down moment.
In both cases the lowered angle of attack restores roll damping on the wing. This is really what stops a spin, yaw is less of a problem here.

Theoretically possible, but major weight and drag penalties for a very rare even. I have worked certification. Of several airliners and we always used an anti spin parachute. But only on a prltotype,the one dedicated to stall development.. Most programmes  if a stall protection system is required may need up to 3000.. or more stalls before certification..And yes wings will drop at the stall  and perhaps authorisation into a spin will start ... .. But we develop the stall.prove tion system to give the customer pilot's margin ... While not impacting performance.


text:
Is there a standard definition of the "stall" phenomena?
In the process of increasing angle of attack of a wing, lift is increasing to a maxima at a certain AOA, then decreasing as this AOA is exceeded.  This seems to be the current agreed upon definition of the stall point.
Yet the sharp pitch down and transition of the center of lift to 50% chord does not occur until the top lift pocket completely collapses.  Is this the true definition of "stall"?
There is also pre-stall buffeting as high AOA turbulence begins to affect the wing trailing edge.
So, are there really 3 parts to this that a pilot must understand, rather than just one?

Your last line asks a different question than your title; which is it? Are you looking for a technical definition of stall or for what does the pilot need to know about it?

Actually both.  A recent ASE question regarding the difference between a "mush" stall and a sharp pitch down piqued my interest.  I have experienced the mushy nose drop in a 172 and have seen very sharp pitch down stalls in staticly unstable rear CG set models.  Thanks for your answer below.

The scientific definition of a stall is as you state in the first line, however in the cockpit that definition isn't actually that useful. Practically there's 2 parts of stalls a pilot needs to be aware of because they are things a pilot experiences, and impact the flying characteristics of the airplane:

Stall buffet: this isn't a stall, but it happens when you're very close to one. Recognizing a buffet may lead to a pilot decreasing AoA before entering a fully developed stall
Flow separation: when the flow separates from the top of the wings it may do so on one wing before the other, causing a wing drop, aileron input may work opposite of intent, and you of course have a high rate of descent. This can feel mild or violent depending on the airplane's characteristics and the speed at which the stall happens

Whether the definition of a stall is when the airflow separates or a change of the center of lift is academic, what a pilots needs to know is how to recognize the condition and recover.

It's basically like oversteering and understeering when driving.  It's not really about the academics, but how it feels and how to recover.

It's a very good analogy to explain the difference between knowing something and experiencing something @Nelson. Skidding is another apt driving analogy.

EASA's unofficial definition:

Stall means a loss of lift caused by exceeding the aeroplane’s critical angle of attack.
Note: A stalled condition can exist at any attitude and airspeed, and may be recognised by
  continuous stall warning activation accompanied by at least one of the following:
(a) buffeting, which could be heavy at times;
(b) lack of pitch authority and/or roll control; and
(c) inability to arrest the descent rate.

A pilot does not need to understand the technical definitions of each stage of a stall. A pilot needs to be able to recognize the indications of the onset of a stall and how to recover from it. This is part of primary training, and it should be part of familiarization training when getting checked out in a new type of aircraft.

I think the distinction between the aerodynamic definition of a stall and a pilot's experience is that while most diagrams show a cross section, the whole wing doesn't always stall at the same time.
Commonly the wing root is designed to stall first, while the tips are still flying. The result is buffeting from the stalled section and 'mushing' due to the reduction in lift but control is maintained.
If the wing is not designed like this, one tip will stall slightly ahead of the other, resulting in asymmetric lift and a sudden roll, probably flicking the aircraft into a spin.


text:
Does Molecular Weight of a Gas affect its lifting properties at the same velocity over the same wing?
Let's say we test the coefficient of lift of the same wing at the same AOA and the same flow velocity at the same pressure.
The difference is wind tunnel A will contain helium (molecular weight 4) gas and wind tunnel B will contain air (average molecular weight around 29).
Which test will produce a higher lift coefficient?  Why?

The wing will produce much less lift in helium than in air; all else being equal, lift is proportional to the molar mass of the gas in question.
The lift equation states that lift $L$ is proportional to the air density $\rho$ (rho):
$$L = \frac12 \rho \ v^2 \ S \ C_L,$$
where $v$ is the true airspeed, $S$ is the wing area, and $C_L$ is the coefficient of lift.
Meanwhile, one form of the ideal gas law tells us how the air density $\rho$ is related to the molar mass $M$:
$$\rho = \frac{MP}{RT},$$
where $P$ is the (static) pressure, $R$ is the gas constant, and $T$ is the absolute temperature.
Combining both of these equations, we find that
$$L = \frac{M P \ v^2 \ S \ C_L}{2 R T}.$$
In words, lift is

the molar mass of the gas,
times the static pressure,
times the square of the airspeed,
times the wing area,
times the coefficient of lift,
divided by the absolute temperature,
divided by a constant (the gas constant $R$),
divided by $2$.


text:
Does the explanation of why slower airfoils are more efficient involve less energy loss from heating or compression of the working fluid?
We read time and time again, how more efficient slower airfoils such as helicopter rotors, props, and sailplane wings are compared with rapidly rotating fan blades or faster (still subsonic wings).  We also read where some people consider compression a negligible factor below a certain speed, even to the point of considering airflow < 200 mph as "incompressible"!
Yet looking at this commonplace and ubiquitous phenomena (indeed, while your prop is putting out 600 lbs of thrust, your wing is creating 4x more lifting force), is there a mathematical explanation for this involving the ideal gas law:  PV=nRT?
Can we visualize creating and maintaining a compressed "bow wave" ahead of a moving surface involves substantial amounts of energy even at lower speeds?
I have yet to fully understand why "it is better to move a lot of air a little than a little air a lot".

I believe the <200 mph thing is more to do with the idea that the temperature rise from compression is low enough to not require temperature corrections between surface and freestream until you are above that speed.

The “it's better to move a lot of air a little than a little air a lot” is because you need to give the air certain momentum to produce the desired amount of lift and if you accelerate less air to higher velocity, you must give it more kinetic energy, which manifests as induced drag. However, the second part of the explanation is form drag and I realized that I am not sure whether it fits your description.

I'm not sure what you're asking...

@Jimmy Let's carry on Hudec's line "you must give it more kinetic energy" - it begins to form a compression wave - won't this happen even a lower speeds?

Related (maybe a duplicate? I'm having a hard time following your question): https://aviation.stackexchange.com/q/53709/4108 and https://aviation.stackexchange.com/q/54808/4108

@Sanchises the answer provided by Kiracofe explained the "move a lot, slower" efficiency advantage.  This is what I was after. Though I remain interested in compression effects at lower speeds, this apparently is not the cause of the thrust vs lift paradox.  I look forward to testing what Hudec and Kiracofe have provided just like James Watt did, only this time lifting the same weight with different airfoils, measuring (electric) power input required.

@Robert, most aerodynamicists consider anything under about Mach 0.7 to be 'low speed', where compressibility can be ignored.

@Robin Bennett I'm getting that for airstreams, am curious about contained spaces such as a cup anemometer.

@Robert, that's not really a contained space. It's in free air and pressure changes move at the speed of sound, so the pressure changes caused by changing speed and direction will dwarf any compressibility. An example of a 'contained space' would be the inside of a piston engine, where compression definitely occurs.

@Robin Bennett "changing speed and direction" hmm, implies some sort of energy transfer, "compression" may not be the best word.  I will continue to study it.

@Robert, good point. Compressibility has a very specific meaning in aerodynamics, linked to shock waves. Pressure definitely does change though, which might explain the confusion.

So you asked a whole bunch of things, and I'm not sure I really understand most of what you are asking, but I can maybe clarify this one part for you

I have yet to fully understand why "it is better to move a lot of air a little than a little air a lot".

The equation for thrust is just Newton's second law F=$\dot m \Delta$V, where $\dot m$ is mass flow rate (kg/s) and $\Delta$V is the change in velocity of the air (exit velocity minus inlet velocity).  So, if you move 1 kg/s of air at 10 m/s, or you move 10 kg/s of air at 1 m/s, the thrust is the same.  Doesn't matter.
But, the equation of kinetic energy (well power really in this case) is $P=(1/2) \dot m (\Delta V)^2$.  So if you move 1 kg/s of air at 10 m/s, it takes $0.5*1*10^2$ = 50 W of power to do that.  But if you move 10 kg/s of air at 1 m/s it takes $0.5*10*1^2$= 5W of power to do that.  Exact same amount of thrust, but with literally 10X less energy required.  i.e. you need to burn way less fuel if you can move a lot of air at a low velocity than if you move a little bit of air a high velocity.

Daniel (and Jan) thanks so much, this perfectly clarifies why a larger, slower airfoil or prop is more efficient +10!

@RobertDiGiovanni, note that this explans the first part, why longer span airfoil has lower induced drag. However, it has lower induced drag at all speeds, so to see why it's optimum point is at lower speed you also need to know why the form and skin drag increase with speed, and I don't actually know any derivation of that.

On your question of why "moving a lot of air a little than a little air a lot", you can use the Momentum Disc Theory, applicable to wing or propulsor. Assuming incompressible flow, the main result is:

Thrust: $T=\dot{m}(u_e-u_0)$
Input power: $P_{in}=\frac{1}{2}\dot{m}(u_e^2-u_0^2)$

where $u_0$ is the free-stream speed, $u_e$ is the airspeed after being accelerated by the wing, propeller or compressor (exhaust speed), $\dot{m}$ is the mass flow rate through the device.
As you can see, thrust is linear to both mass flow rate and exhaust speed, but input power is square of exhaust speed. That's why it's more efficient to move a lot of air a little than a little air a lot.
On your question of compressibility, for airspeed less than Mach 0.3 (around 200mph at sea level), there isn't much to be gained by assuming the flow is compressible. Yes, air is compressible, but so is everything else in the world. The difference is, if an air molecule is moving slow enough, the air molecules ahead of it can get out of its way without being squished. And the mechanism has to do with speed of sound. The faster it is, the less "time" they have to do that. Super dumbed down, but that's the intuition.
For a mathematical proof, you can follow this NASA page, whose result I will cite here. Assuming a compressible, isentropic, non-viscous fluid, we have:
$$-M^2\frac{du}{u}=\frac{d\rho}{\rho}$$
So for M=0.3, 1% change in airspeed is only 0.1% change in density. For most engineering purposes, negligible.

So unless you "trap" the higher pressure over a meaningful area (as in a heavily undercambered wing or parachute) or you go pretty fast, air flow CAN be modeled as incompressible, and pressure/drag contributions are negligible.  Fine work!

@RobertDiGiovanni I believe even for parachute, incompressible is a good assumption unless we are talking about high speed spin chutes. However, you can no longer assume irrotational and inviscid.


text:
Would a canard work better for emergency pitch down?
In order for a conventional horizontal tail plane to generate a nose pitch down effect in a stall condition, it must increase its angle of attack even more (by use of elevator or pitching up more).  A front mounted canard would pitch DOWN into the airstream, increasing chances of remaining unstalled. A canard would also be in relatively undisturbed airflow.
The old fashioned solution is to mount a lower aspect, wedge , or delta shaped horizontal horizontal tail that clearly allows the main wing to stall first.  Newer designs, especially in airliners, feature reduced size Hstabs of similar planform to the wings, relying on computers to maintain pitch control.
Would a downward pitching canard be a better solution to avoid deep stall or stalls in general?

Tail stall at positive AOA generally is not a concern. Tail stall is a concern for down load, in zero-G pushover, for example.

@Jimmy and is not a "zero G pushover" requiring tail UPload.  See what can happen in deep stall?

It's the force reversal following the push over.  There were several Twin Otter crashes caused by pilots doing a zoom climb following takeoff followed by a hard push over that put the airplane semi ballistic.  The pitch rotation got high enough that the local AOA at the horizontal tail exceeded the stall angle, and the tail more or less completely quit lifting down when downforce was called for.  Being only at about 100 ft at this point, the airplanes pitched straight into the ground.

@RobertDiGiovanni  are you proceeding from the notion that the horiz tail lifts in steady state flight and needs to be able to keep lifting after the main wing stalls to ensure a pitch over?  Conventional airplanes pitch over at the stall because the tail's downforce can no longer overcome the main wing's pitching moment at the stall because of the sudden aft CP shift and down the nose goes.  Airplanes with regular tails, like the Ercoupe, were made "stall proof" simply by limiting elevator authority to limit downforce near the stall.  The tail simply couldn't push down hard enough.

@John K I am proceeding from the notion that large swept wing aircraft can deep stall, and post stall, the computer won't know which way to pitch to unstall due to altered airstream near the tail.  With a canard there would be no confusion with pilot or computer, pitching down (much like the foreplanes of a submarine) would lower the nose.

@John K and yes, that tail needs to lift to pitch over, especially if the weight is not a little forward.  The tail lifts 2 ways, by deflecting the airstream as an airfoil, or by simply weather vaning from change in relative wind as the plane sinks.  Critical in large aircraft is to reliably pitch down BEFORE stall, a canard would work every time.

A couple of confusion here. For starters, unless we are dealing with T-Tails, there is no deep stall for a tailed airplane. On the other hand, a canard airplane has the potential to enter deep stall.
Next, for a transport category aircraft, tail stalling in the positive incidence is generally not a concern. Tail stalling in the down load, however, is; for example, zero-G pushover in icing condition is a real issue to consider during design. In a well designed aircraft, the tail would have a higher sweep and lower aspect ratio than the wing so that the wing stalls earlier than the tail.
(Zero-G pushover may stall the tail because of the pitch rate build-up. Nose-down pitch rate equals negative flow incidence on the tail. Once the tail stalls, you lose the ability to pull.)
It could be an issue, however, if you have a T-Tail. For a swept wing in cruise configuration, the pitching moment would likely reverse near stall. At that point, the elevator would still have good effectiveness, but depending on how aggressive the pitch up is, there may not be enough time to lower the AOA before the aircraft enters deep stall.
Once in the deep stall, the authority on the tail as a whole decreases dramatically. Even with full nose-down stabilizer and nose-down elevator, you may not have enough authority to break the stall. In that situation, without an anti-stall chute, it's game over. The point is, you don't want to ever get there. Preventative measures include stick pusher and envelope protection functions.
Would installing a canard help? Sure, more authority always helps. But does it warrant the complexity and weight increase when other solutions exist? Probably not.

good answer.  But pitching moment of tail, by design, reverses from down to up in ALL cases where AOA increases beyond a preset limit (trim).  (It also reverses when elevator is used (obviously) or when CG is too far back to begin with).  Yes, a larger tail, as another writer pointed out, is another solution, as Tupelov implemented after the Trident deep stall crash. My query was inspired by the B ONE.

Also, the canard would be moveable, used in conjunction with a conventional tail.

@RobertDiGiovanni Well your AOA is increasing because you are commanding it to increase. While untrimmed tail load gets increasingly more positive with increasing AOA, if you are commanding the nose-up, then pitch-rate aside, it is in trimmed condition. Have you seen incidences where a conventional, non T-Tail aircraft has tail-stalled pitching up?

The early F-86 Sabre jets did, and they solved with slats! It's all about pitch authority, as you say.  The Sabres supersonic tail forget lessons from 40 years earlier, when low speed maneuverability ruled.  Many gliders have T tails (their high location saves a tiny bit of drag by pitching nose up without the need to deflect a lower tail).  Slats also are in slightly more predictable airflow, better for dumb computers and pilots like me.

@RobertDiGiovanni I'm not familiar with F-86. Do you have a source on what the deficiency is and what it has to do with your question? What does slats have to do with tail stall in up-load?

James, they help push the nose down.  Loss of lift at the swept wing tips creates the same effect.  Pitch control can be in many places.

@RobertDiGiovanni Sources please. I don't know what effect "loss of lift at the swept wing tips" you are referring to. You made a bunch of claims; some of which are misplaced, others you haven't substantiated.

The effects of insufficient lift for any reason is a concern for a control surface. Reliable pitch control is essential.  I really don't want to argue, my point is a computer or pilot will have difficulty dealing with variable airstream effects, just look at a post stall AOA curve.  That is why I am trying to put emergency pitch control elsewhere rather than right behind the wing.  What do you think?


text:
What guidance is there for pilots when reporting braking action?
I was listening to "[REAL ATC] Eastern B737 (candidate Mike Pence) OFF RUNWAY at LaGuardia!"(1) and the LGA Tower asks for and receives reports of braking action from several landing pilots. I assume that this was during inclement weather, such as snow. Several pilots all report braking action as "fair" for their types.
How is "fair" determined? Is it totally subjective, or is there an objective measure? What are the classifications (of braking action) that a pilot can report?
 (1) Please note, I did not tag this as LGA, since the question is meant to be a general one, unless each reporting is airport specific.

The FAA issued a Safety Alert for Operators (SAFO), "Runway Assessment and Condition Reporting" #16009 on August 15, 2016. In it, it says that:

You'll notice it doesn't say "Fair" anywhere on there, that is because this SAFO replaced the word "Fair" with "Medium" on October 1, 2016.

Pilot braking action reports will continue to be solicited and will be used in assessing braking performance. Effective October 1, 2016, the terminology ”Fair” will be replaced by “Medium” and pilot braking action reports will now describe conditions as Good, Good to Medium, Medium, Medium to Poor, or NIL. This will harmonize the NAS with ICAO standards.

(Emphasis mine)

Since the accident with Mike Pence happened on October 27, 2016, if a pilot was reporting "Fair", they were in violation of the SAFO and should have been reporting "Medium". You can read the NTSB report of this incident here.

“In violation of a SAFO”???  That isn’t a regulatory document.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Could in flight folding wing tips lead to a safer airliner design?
As a spin-off to the recent success of the Boeing 777x maiden flight, the effects of folding the wing tips in flight were considered.
Folding them down in cruise, like the XB-70 should increase yaw stability and create a slight anhedralling effect, which reduces Dutch Roll tendency.  More importantly, it would shift the Center of Pressure forward, eliminating the need for downforce from the tail.  This creates a desirable, fuel saving, staticly nuetral configuration.
But the real safety improving benefit would be to fold them down on approach to landing, requiring downforce on the tail to balance the lifting forces.
Would this not create a safer, staticly stable configuration, where approach speed can be trimmed?

Can you think of a particular accident that this might have prevented?

@Terran Swett do you need an accident before improving safety?  Folding the wing tips (for swept wings) moves the CP forward, eliminating the need for tail downforce.  This is cruise configuration.  The safety comes in converting back to tail downforce static stability on approach and landing.  You can now trim for speed.  This will be helpful on take-offs too.

@RobertDiGiovanni airliners are already longitudinally stable, do you want more stability? Why? Excessive stability is not necessarily a good thing.

@AEhere supports Monica  longitudinal stability and static stability are 2 different  considerations. Static stability is undesirable in cruise, so "modern" aircraft set CG further back (but not so far back to be directionally unstable) to be staticly nuetral, which avoids drag penalty.  The wing tip idea (for swept wings) is to move CP forward so tail downforce is not needed in cruise.  On approach, wingtips are folded down and elevator downforce applied, creating a "staticly stable" configuration that can be trimmed for speed.

@RobertDiGiovanni " longitudinal stability and static stability are 2 different considerations" well, yes, in that each of those terms is independent of the other: you can have longitudinal static stability, for example. "Static stability is undesirable in cruise" are you sure of this? Or do you misunderstand the terminology? Because I'd say having an aircraft that does not try to pitch further into a dive is in fact, desirable.

@AEhere supports Monica that would be staticly unstable.  Ideally, in cruise, one wing carries all the lift, not 2.  A plane set up for static stability has some downforce on the tail, adding drag and "weight". The staticly unstable set up has upforce on the tail, slightly more efficient, but, as you say, less desirable.  Neutral static stability, with very sensitive computer control is most efficient (maybe a hair on the stable side).  This is why I proposed a sliding cargo bay for the C-130 Instead of a humongous tail.

@RobertDiGiovanni so your previous comment was a typo then? Longitudinal static stability is desirable the vast majority of aircraft without artificial stability systems, and even in some with it, for safety purposes.

@AEhere supports Monica really surprised, in light of current events, you would make such a proclamation.  Did you read my question?  It's about airliners, who "relax" static stability to save fuel.  The proposal is to be staticly stable for take-off and landing and staticly neutral in cruise.

@AEhere supports Monica so, get a paper clip and a piece of paper and build a nice stable delta glider.  Then cut little elevators in the back and move the weight around to see static stability.  You will find a staticly unstable plane has pitch stability, but only at its trim speed.  These make great throwing gliders, because they pitch up as their speed decays, resulting in a straight line flight before stall.  If you have a hill handy, staticly stable gliders give you a great show all the way down.

A few issues make fold-up wing tips unfeasible:
Weight and complexity of a mechanism and control system for it.
Shifting CP forward makes the airplane less stable statically. In any case, if you want to reduce trim drag, just move the C of G aft to unload the tail to get the same result, and use computers to artificially induce static margins, which is already done.
The stall speed reduction from the extra wing area is quite small.  A wing tip that added  10% to wing area when dropped would only drop the stall by a few knots.  The stall speed reduction from adding flaps dwarfs the reduction you get from adding area.
At high altitudes, you want the extra span so you wouldn't want to retract the tips anyway.  On an airliner that cruises sub-sonic at high altitudes, there's no benefit to reducing span.  On sub-sonic airplanes, where you clip off the wing tips, it's to improve speeds at low altitudes (like the clip wing Spitfire, done to improve performance below 20000 ft - for the very high altitude versions used to take on JU-86s, they added span).
The only practical reason to have folding wing tips on an airliner is to reduce span on the ground, as an alternative to winglets (which are used to gain some of the benefit of added span without taking up space or adding to bending loads on the wing), but a winglet is a much cheaper solution.
Which, as always, is why you haven't seen anybody do folding wing tips in the last 60 years, whereas winglets are everywhere.

Yes, but using computers is so old fashioned!  And the 777 has a bit more power than the JU-86. I had worried about me mechanically moving the wing tips, then realized they could be unlocked and flown into position with aerodynamic force.  Once in position they become .... winglets!


text:
What would the point to point range of the current Virgin Galactic Spaceship 2 launch system be?
Many people have suggested that the next step for Virgin Galactic after single point "space tourism" flights begin would be passenger suborbital flights, providing travel times many times faster than even the Concorde.
If they flew up to perhaps 30 miles, instead around 60 miles, much more of the rocket energy could be devoted to moving Spaceship 2 downrange.
If the Knight carrier aircraft flew straight towards the destination, how many miles would it travel before it reached the 50,000 foot drop altitude?
Then, on a flatter trajectory, how much further would Spaceship 2 go? How far could it glide after re-entry?
Not to rush things too much, but a "pilot scale" point to point flight from the Spaceport in New Mexico to California might be a good starting point.  Would it be possible?

This is going to involve a lot of speculation!
I'd guess that White Knight Two will have to fly at approximately airliner speeds once it reaches altitude, otherwise it will stall in the thin air. Total flight time of the combination is expected to be 2.5 hours, which gives a range of about 2500km (if we optimistically assume it does Mach 0.8 the whole time, the initial climb is likely to be much slower)
Spaceship Two is projected to have 6 minutes of freefall and a top speed of 4000 km/h, so that's another 400km. If they thrust at an angle rather than straight up the top speed may be higher but the flight time will be lower, so I think it's a reasonable estimate.
If it starts gliding from 50,000ft (15km) and has a glide ratio of 5:1, it can cover another 75km (so picking a glide ratio out of the air has very little effect on the total)
I doubt anyone will pay $250,000 to fly 3000km at basically the same speed as an airliner.

Would it not have a greater range if it reaches the highest altitude possible? Less aerodynamic drag. Perhaps a 10% extra of fuel can make the suborbital portion would sustain a greater distance, rather than burning most of it going horizontal at low (50k) altitude.

@Cody - Spaceship Two can't glide when it's above the atmosphere, and it's not designed for efficient, rocket powered, supersonic cruise. There are some ideas for doing this, known as 'waveriders' but it's not clear if they're practical or would require as much fuel as reaching orbit.


text:
Is there any System to warn of Runway Incusion?
From what I learn, in large towered airports, the control of who enters and leaves the runways belong to the Control Tower Operators.
And that got me thinking:

Is there any Equipment or System in large airports to warn the tower of something/someone entered or is crossing the runway?

In any (not just large ones) towered airport the control of who enters and leaves the runways belongs to the tower. That is a point of having a tower.

Yes, there are several systems.
The simplest are REALLY BIG WINDOWS (and binoculars):

When the entire wall in front of you is a big expanse of glass looking out over the runway there's a good chance you'll notice something trying to roll onto the runway.
Many runway incursions are detected visually.
Really big windows don't work as well when you have multiple runways and larger airport complexes though, so we have some technological solutions too. The most common is Automated Surface Detection Equipment (ASDE), the one being deployed today is ASDE-X.
This is basically "radar on the ground" which interrogates aircraft transponders and shows the controllers which aircraft are where on the airport surface:

There are also systems on the ground designed to prevent incursions, but those are mainly for use by pilots.
These include Runway Status Lights, taxiway guidance lights (which include "stop bars" that light up on hold-short lines), and "runway guard lights" (which may be mounted either in the pavement or alongside the taxiway/runway intersection).
Many of these systems can be controlled from the tower, either by automatic systems or manually by the controllers.

Those 3 links at the end of your answer go a long way toward answering my developing question about runway/taxiway lighting. Very informative, thanks!

with ASDE do ground vehicles get fitted with some kind of simple transponder to make them more visible to the system? I imagine a tug might have a fairly small radar cross section. Or would that not pass cost / benefit?

@TomMcW Yes - ground vehicles on airports with ASDE systems are usually fitted with a "ground vehicle transponder" to report their location to the system. This probably doesn't cover every possible ground vehicle (e.g. the luggage truck that stays on non-movement areas may not have a transponder), but airport operations vehicles and ARFF equipment will be equipped, for obvious reasons.

In addition to warning for taxiing pilots, there are FAROS (Final Approach Runway Occupancy System) indicators approved for evaluation at certain airports in the U.S. This system will flash the glide slope lights (PAPI/VASI) if an aircraft is detected to be in the way of an arriving aircraft, which alerts an approaching pilot to go around if he's below decision height. Incursion can be determined by a system as cheap as an inductive loop in the asphalt under the threshold and numbers (same as the technology for controlling on-demand traffic lights), or by IR sensors, ASDE, etc.


text:
SR-72 --- is the speed reported in this story wrong?
I was just reading this story from Sky news and reported the following

Lockheed Martin says it is close to developing a military plane that flies at Mach 6 - six times the speed of sound, which travels at around 762mph (1,236km per hour).

Am I missing something, or is that a misprint?
6 * speed of sound at sea level = 4567.2423 miles per hour

The speed of sound changes with temperature (and thus usually with altitude), see:
http://aviation.stackexchange.com/questions/3693/how-does-altitude-affect-the-speed-of-sound

They have quoted the speed of sound at sea level - but planes don't cruise that low!

No, that's not a misprint, that is a bad writing. The subordinate clause relates to “sound”, not to the plane, so the number would be right if speed of sound at room temperature was relevant.

I thought that the general unit of measure for true airspeed was knots, whereas Mach is measure in units relative to the speed of sound for a given temperature, pressure, and air density.  ????

is the speed reported in this story wrong?

No
No it is not a misprint that the SR-72 is intended to be capable of Mach 6
From the horses' mouth:

Envisioned as an unmanned aircraft, the SR-72 would fly at speeds up to Mach 6, or six times the speed of sound. At this speed, the aircraft would be so fast, an adversary would have no time to react or hide.
The SR-72’s design incorporates lessons learned from the HTV-2, which flew to a top speed of Mach 20, or 13,000 mph, with a surface temperature of 3500°F.

Yes
Yes it is not true that Mach 6 is necessarily 4567.2423 MPH
From elsewhere:

Cmglee. License:CC by SA 3

Although as noted in that Lockheed article, the SR-71 did 5,600 km London-New York in <2 hours. With the SR-72 meant to be twice as fast by the same metric (Mach 6 vs Mach 3), that would indeed suggest Lockheed are talking about a ground speed of at the very least 5,600km/h. This is on the assumption that Lockheed are using the same definition of "Mach" for both aircraft, and suggests they're talking about the speed of sound at sea level.

@JonStory: So far as I know, in Aviation, Mach is always defined in terms of the actual altitude of the aircraft. The local speed of sound at a specific altitude has a dramatic aerodynamic effect on the aircraft. Otherwise most ordinary airliners would be travelling subsonically at Mach > 1.

Yeah totally agreed... but when communicating with the public people expect the scale to be linear. So in the aircraft I doubt it will say "Mach 6" on the ASI, but when announcing it to the public it appears Lockheed are sticking with sea level, so they can compare with the SR-71, Concorde, F-15, Boeing 737 etc

@JonStory: Lockheed wrote "Mach 20, or 13,000 MPH". 13000/20=650 not 762.

so, less air resistance = faster flight at low speeds. ?

@RedGrittyBrick -- now i understand :)

@RedGrittyBrick: It's temperature, not density that determines the speed of sound.  Colder air = lower speed of sound. The break at 36K feet is due to the tropopause above which air temp is relatively constant.  Look at the correlation of the curves on the chart.

@Gerry Very true, we even have a whole question and answer devoted to it: http://aviation.stackexchange.com/questions/3693/how-does-altitude-affect-the-speed-of-sound

While I'd in general remain sceptical of anything that Sky News talks about the numbers do seem realistic. Aviation Week (amongst others) have mentioned similar speeds and they're not beyond the realm of likelihood. Hypersonic test vehicles have already reached above Mach 5 and many of the designs are capable of the speeds talked about in the article. It's also worth noting that the SR-72 wouldn't enter service for another 15 years.
It's impossible to say for definite, though, as we're discussing something that is: a) In the design stage and b) Very secretive.